1. Field of the Invention
The invention relates to a method for moving at least one guide head of a wire erosion machine, possibly a workpiece, along at least one guide path. The invention also relates to a wire erosion machine having an actuating device for executing movement of at least one guide head, possibly a workpiece, on at least one guide path. A machine or a method, respectively, of this type is known from U.S. Pat. No. 4,546,227 or from EP-0 068 027 B1.
Spark or electroerosion is a phenomenon wherein dielectric discharge occurs between two electrodes across a dielectic, resulting in a removal of material at one of the electrodes. This method is used since many years and increasingly for machining of metals since it permits fabrication of very concise contours. In respective machine tools, the so-called wire erosion machines, one electrode is formed by the workpiece, and the other electrode by a tool--called tool or die electrode. For attaining the desired contour, generally a numerically controlled relative motion between the workpiece and the two electrodes is provided. A flushing agent, usually water, which flows around the erosion zone under a certain settable flushing pressure serves as dielectric between two electrodes.
The electrode in a wire erosion machine is a cutting wire which is continuously unwound from a wire supply and guided via a first wire guide above the workpiece to the erosion zone and pulled from the erosion zone over a second wire guide below the workpiece into a receptacle for disposal. When a preset contour is to be cut in the workpiece, a precisely controlled relative motion between the cutting wire and the workpiece is required. The relative motion is usually realized by numerically controlling the movement of the workpiece and/or of the guide heads supporting the wire guides. This relative motion has to ensure that the erosion zone in the workpiece moves along a path--the contour curve--which follows the contour to be machined as accurately as possible.
Ideally if the cutting wire would travel in an exact straight line between the upper and lower wire guide, then the relative movement between the wire guide head and the workpiece would have to be matched, for example for a cylindrical cut, exactly the aforementioned contour curve. Although the wire is tensioned by drive and brake rolls in the guide heads, a certain bulging of the cutting wire opposite to the cutting direction cannot be avoided, which is caused, in particular, by the flushing and erosion pressure in the erosion zone. This bulging causes problems when the contour to be cut on the workpiece changes direction, especially at sharp curvatures or corners of the contour curve. This is the point where the so-called drag error manifests itself, which tends to flatten the contour curvature and to round the corners. This phenomenon is about comparable with the error between the tracks of a pulling and a pulled vehicle which can be observed when the vehicle rounds sharp corners.
2. Description of the Related Art
Different measures for compensating the drag error are known. For instance, U.S. Pat. No. 4,546,227 and EP-0 068 027 B1 disclose different methods whereby the bulging of the wire is measured by suitable position sensors while the erosion process is interrupted, and whereby control variables are calculated from the respective measured values which effect the above-described relative movement by minimizing the effects of the drag error. In U.S. Pat. No. 4,546,227 there is disclosed a method wherein for cutting a corner, the relative movement between the cutting wire and the workpiece is temporarily halted, and the drag error is measured. In EP-0 068 027 B1 there is described a method wherein two correction values are calculated from the measured wire bulging, with one of these correction values relating to the tangential and the other value to the radial component of the drag error at or near a curvature. These correction values are used to correct the relative motion of the advance system which was stored in a memory for the contour to be cut, by compensating the drag error. This means that the actual path which is effected by the advance system, of the relative movement between the wire guide heads and the workpiece is displaced--depending on the measured values for the wire positions--with respect to the contour curve to be cut.
Each of the two methods described above have different disadvantages, so that they cannot be employed universally. For instance, the first method described above which executes a edge strategy by reducing the erosion parameters, is very time consuming. The average cutting rate is reduced here depending on the radial content of the workpiece. For example, for a workpiece with a 15% radial content, only half of the maximum cutting rate is achieved.
With the above-described second method, the erosion speed does not have to be reduced because the guide path of the guide head is continuously corrected, thereby allowing the average cutting rate to be equal to the maximum cutting rate.
On the other hand, the decomposition of the drag error vector on an arc of a circle into the tangential and the radial component is static and independent of the direction of the drag error at the origin of the path element. There is no further correction provided upon return to a straight cut or at a transition from one straight cut to another straight cut. The static computation breaks down especially for small radii and for large changes in direction.
From EP-0 312 056 there is known a "sensor-supported" measuring device for a measuring the displacement of the wire electrode. The relative position between the guide head of the wire electrode and the workpiece can here be corrected in such a way that machining errors caused by the wire displacement can be prevented. This method has certain limitations which have to be taken into account by the operator. Especially under difficult operating conditions, i.e. if, for example, the contour to be cut is too close to the edge of the workpiece or contains cutting paths which are too close to each other, the measurement of the displacement can be unstable.